1. Field of the Invention:
This invention relates to an apparatus for preventing the vertical displacement or "rise" of an ink ribbon in a dot-line printer.
2. Description of the Prior Art:
Many types of printers are available and are used as output units for computers. Of these types, the use of a dot-line printer recently has shown a rapid increase in popularity. This type of printer has several desirable features including a high degree of font flexibility and a high printing speed. Prior art dot-line printers, however, are prone to ribbon rise.
The pertinent structure of a prior art dot-line printer and the problem of rise of an ink ribbon will hereinafter be discussed with reference to FIGS. 1 to 3.
The dot-line printer is a type of printer having printing hammers 11 designed for the dot printing of letters of figures each composed of a dot matrix. A hammer unit 10 carries a plurality of printing hammers 11 which are spaced laterially apart from one another along the direction of the line to be printed and are reciprocally movable. Each printing hammer 11 comprises a leaf spring formed with a printing pin 12 positioned at one end thereof. This leaf spring is disposed in an electro-magnetic sub-assembly which comprises a permanent magnet 13, a generally L-shaped yoke plate 14, a comb yoke 16 provided with a release solenoid coil 15, and a front yoke 17.
The comb yoke 16 is secured to the yoke plate 14 by bolts, not shown. The front yoke 17, the printing hammers 11 and the permanent magnet 13 are secured to the yoke plate 14 by a bolt 18. The printing hammer 11 normally is held in a bent form against one end of the comb yoke 16 by the attractive force of the permanent magnet 13. The solenoid coil 15 is energizable to enable printing. If the solenoid coil 15 is energized, the attractive force of the permanent magnet 13 is overcome, and the printing hammer 11 is released from the comb yoke 16, so that the printing pins 12 may strike ink ribbon 20 and paper 30 against a platen 50. The ink with which the ink ribbon 20 is impregnated is transferred onto the paper 30 to effect dot printing. The hammer unit 10 is supported movably in the direction of the line to be printed by a shafft 62 provided with a slide bearing 19 at each end thereof, and secured to a mechanical frame 60 by bolts 61. The hammer unit 10 is reciprocally movable in the direction of the line to be printed by a drive system, not shown. The distance of such reciprocal movement is at least equal to the distance between two adjoining printing hammers 11. The paper 30 is drawn through the printer from below upward by a tractor, not shown, for each printed dot line. The reciprocal motion of the hammer unit 10, the feeding of the paper 30 and the driving of the printing hammers 11 are selectively controlled as to enable the printing of the desired letters and figures each composed of a dot matrix. The ink ribbon 20 is in the form of a tape as shown by the two-dot chain lines in FIG. 2, and travels in front of the front yoke 17 generally in the direction of the printed line. More specifically, the ink ribbon 20 travels at an angle to the printing pins 12 which are aligned together in the hammer unit 10, as shown in FIG. 2. This allows the ribbon 20 to be used more efficiently resulting in prolonged ribbon life. The hammer unit 10 carries a main ribbon separator 21 disposed between the ink ribbon 20 and the paper 30. The main ribbon separator 21 comprises a thin stainless steel plate, and is provided to deter the rise of the ink ribbon 20 when the paper 30 is fed upward. The main ribbon separator 21 also serves to prevent any contamination of the paper 30 by the ink ribbon 20.
The main ribbon separator 21 has apertures 26 through which printing pins 12 pass to effect printing. A duct is provided above the hammer unit 10 for cooling the solenoid coil 15. A hammer unit cover 70 is connected to the frame 60 by a stand 63. A paper press holder 71 is provided above the hammer cover 70 for stabilizing travel of the paper 30. The hammer unit cover 70 has a side facing the platen 50 and located close to the front yoke 17, as shown in FIG. 1. The ink ribbon 20 can easily be set in position without having its lower edge interferring with the adjacent end of the front yoke 17. The ink ribbon 20 travels between two spools, not shown. Ribbon travel may be reversed alternately to travel from the left to the right and from the right to the left in a manner well known in the art.
In the dot-line printer hereinabove described, the problem of ink ribbon rise is not completely eliminated by ribbon separator 21. The rise of the ink ribbon 20 means it moves vertically relative to its desired position displacing it from the position in which it faces the printing pins 12 on the printing hammer 11. This results in printing omissions which is unacceptable. One of the primary reasons for this trouble is the presence of perforations 31 formed in continuous feed-type paper 30 for dividing one page from another. Typically, perforations 31 project toward the hammer unit 10 as shown in FIG. 1; however, they may also project in the opposite direction. When the perforated portion of the paper 30 passes between the platen 50 and the printing pins 12, the ink ribbon 20 is compressed between the ribbon separator 21 and the front yoke 17. This creates a very large resistance to the ink ribbon 20 travel in front of the hammer unit 10. This resistance increases with an increase in the thickness and type of the paper 30. Under ordinary circumstances, the ink ribbon 20 usually has an adequate degree of tension to insure that it travels along the desired path while it is travelling. The presence of perforations 31 of the paper 30 between the platen 50 and the printing pins 12, however, creates a larger resistance than such ribbon tension, especially if the paper 30 is thick. This resistance causes the ink ribbon 20 to vary in the direction of its travel. This destroys the stability in the travel of the ink ribbon 20. The ink ribbon 20 travel sometimes is caused to move up and down, and its lower edge is caught by the corner 17' of the front yoke 17 as shown at circled portion A in FIG. 2. If the ink ribbon 20 is traveling from the left to the right in FIG. 2, it progressively enters the clearance between the upper end of the front yoke 17 and the hammer cover 70, and moves away from the printing pins 12 on the printing hammers 11, as shown by two-dot chain line in FIG. 3. This results in total printing omissions. This trouble can be avoided if the front yoke 17 is enlarged so that its corner 17' may extend to a higher position. This method is, however, undesirable for a number of reasons, including difficulty in the manufacture of the front yoke 17, an increase in the mass of the hammer unit 10 and interference with hammer unit cover 70.
Another problem has already been pointed out. Although the clearance between the upper end of the front yoke 17 and the hammer cover 70 is very small, the ink ribbon 20 only has a thickness of 1 to 2 mm. Consequently, the ink ribbon 20 can readily enter the space between the front yoke 17 and the hammer unit cover 70. The ink ribbon 20 is, therefore, difficult to set properly in position.